Class 10 Science Chapter 1: Chemical Reactions and Equations worksheet with answer key
A 20-mark worksheet on Chemical Reactions and Equations for about 40 minutes of class or homework: 5 multiple-choice, 3 very short answer and 3 short answer questions, written from the NCERT chapter and checked by a second reviewer. Print the PDF for the class; the answer key carries the marking scheme. Teachers may photocopy it freely.
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Answer: (c) Slaked lime brushed on a wall turns into a hard, shiny layer over three days.
Bending, boiling and shattering change only shape or state. On the wall, calcium hydroxide reacts with carbon dioxide of the air to form calcium carbonate, a new substance.
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Answer: (a) changed the formula of the product instead of changing coefficients
A formula is fixed; only the coefficients in front may be changed. Al₂O₂ is not aluminium oxide. The correct equation is 4Al + 3O₂ → 2Al₂O₃.
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Answer: (a) calcium carbonate
Calcium hydroxide on the wall slowly takes up carbon dioxide from the air to form calcium carbonate, the shiny layer. A carbonate with dilute acid gives carbon dioxide, which turns lime water milky.
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Answer: (a) calcium hydroxide; exothermic
Quicklime (CaO) and water form slaked lime, calcium hydroxide, Ca(OH)₂, with a large release of heat, so the reaction is exothermic.
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Answer: (b) Zinc + Copper chloride → Zinc chloride + Copper
A word-equation names the reactants and products; the others use chemical formulae, and (d) is not a real reaction at all.
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(a) Heat is released, so the decomposition of vegetable matter into compost is an exothermic reaction. (b) Respiration: glucose combines with oxygen in the cells to give carbon dioxide, water and energy.
- exothermic, because heat is released (1)
- respiration as the exothermic reaction in the body (1)
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KOH + HNO₃ → KNO₃ + H₂O. An acid and a base give a salt and water: a neutralisation reaction, which is a double displacement reaction.
- balanced equation giving KNO₃ and H₂O (1)
- neutralisation (a double displacement reaction) (1)
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(a) The plant needs energy from sunlight to turn carbon dioxide and water into glucose; without light the reaction cannot go, so it absorbs energy: endothermic. (b) 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O, with 'sunlight, chlorophyll' over the arrow (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ also accepted).
- energy from sunlight is absorbed, so endothermic (1)
- correct equation with sunlight and chlorophyll over the arrow (1)
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(a) The brown powder is copper and the solution now holds zinc sulphate. Zinc is more reactive than copper, so it displaces copper from copper sulphate. (b) Zn + CuSO₄ → ZnSO₄ + Cu (Zn 1, Cu 1, S 1, O 4 on each side). (c) No. A displacement needs a metal more reactive than the one in the salt; copper cannot displace copper.
- copper and zinc sulphate, zinc being more reactive than copper (1)
- Zn + CuSO₄ → ZnSO₄ + Cu (1)
- no change in copper, with the reason (1)
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(a) Nail X, which gets coated with a reddish brown powder. (b) Corrosion of iron, called rusting. (c) Air (its oxygen) and moisture; the oil and paint cut both off from the iron surface.
- X, with a reddish brown coating (1)
- corrosion (rusting) (1)
- air and moisture, both named (1)
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(a) P gives hydrogen; Q gives carbon dioxide. (b) A lit splint brought near the gas from P goes out with a pop (hydrogen); the gas from Q turns lime water milky (carbon dioxide). (c) CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
- hydrogen from P and carbon dioxide from Q (1)
- pop with a lit splint, or lime water turning milky (1)
- CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ (1)